Definition of Human chromosome count

Human chromosome count: The usual number of chromosomes in humans, 46 chromosomes. Also called the human chromosome complement.

History: For half a century, the human chromosome count was widely believed to be 48. That changed on Dec. 22, 1955. On that day at the Institute of Genetics of the University of Lund in Sweden, a visiting scientist named Joe Hin Tjio (pronounced CHEE-oh) used what was then a new technique to study the chromosomes of cells on a glass microscope slide. The technique employed the drug colchicine and a dilute salt solution. Colchicine froze the cells at a point during division when the chromosomes had separated into clearly visible bodies. The dilute salt solution (hypotonic saline) spread the chromosomes so they could be seen separately from one another.

Tjio succeeded in obtaining a clear picture of the human chromosome complement on a photomicrograph (a photograph taken through a microscope) of a human embryonic lung cell. Much to his amazement, Tjio discovered that the cell had 46 chromosomes. "The number was just an incidental finding, like serendipity," he recounted. "I was just surprised that it was not 48, as they had thought for so many years."

Tjio published his discovery together with Dr. Albert Levan a month later in the Scandinavian genetics journal Hereditas. Levan, who was the director of the Institute of Genetics, traditionally listed himself as the first author on any publication based on work done in his laboratory. But Levan had been on vacation when Tjio made his discovery. Tjio said "I told him, 'No.' I wouldn't allow him to be first author. I said, 'If you want to be the author, you do the work.'" Levan acquiesced. The now-classic citation is: Tjio TH, Levan A. The chromosome number of man. Hereditas 1956;42:1.

Joe Hin Tjio (1919-2001) was born to Chinese parents in Java, then part of the Dutch East Indies, in 1919. After World War II, Tjio did cytogenetic research in the Netherlands, Sweden and Spain. He came to the United States in 1958 and in 1959 joined the staff of the NIH where he spent the balance of his long career.

Based on obituaries of Dr. Tjio by Bart Barnes in The Washington Post (Dec. 4, 2001) and by Wolfgang Saxon in The New York Times (Dec. 7, 2001) and on personal recollections.